Revisiting the trailer and thinking about it again, it does indeed seem to be more thought provoking than I first thought. At first I didn't see the conflict between "who is the real monster", but when I think about it does seem pretty plausible.
The Polish version of the lullaby makes it very explicit it is a lullaby sung by monsters and so she was obviously raised fearing Witchers, this then means that when she is made aware of Geralt standing behind her, she is approaching the situation on the assumption that he is the monster from the lullabies she's heard, she's approaching it from the point of view of a murderer approaching her. He is approaching the situation from the tales he's heard from the villagers, as far as we know Geralt doesn't actually know she is the monster [in terms of actions] the villagers might have made her out to be, they are both operating on pre-concieved notions of each other's nature, they're both blinded by it.
I think perhaps he realises this in the end of the trailer and that is why he doesn't take a part of her as a trophy, I don't think it's a question of her regenerating, I think he probably thinks she is dead, it's simply disillusionment with what he's done. Sorry, I know that's all obvious to most by now, I just think it's really great following a closer look.